Fiber aids digestion and overall digestive system function. However, the immunoregulatory roles of fiber-rich diets and the metabolites or products produced by the microbiota remain unclear. Microbiota stimulates the immune system and aids in the breakdown of food compounds. Fiber diets are different, and the effects on the immune system cannot be generalized; therefore, mechanisms of microbiota-derived metabolites are undefined.
David Artis, from Weill Cornell Medicine, and colleagues isolated a previously reported inulin fiber diet. Inulin is a commonly used prebiotic that sits in the intestine to help beneficial bacteria grow. Artis and colleagues chose an inulin diet due to evidence that inulin shifts intestinal tract bacteria in a wide range of clinical studies. Artis and colleagues demonstrated that an inulin diet alters murine microbiota and the metabolite products the body can use for nutrients or energy. The microbiota shift was associated with type 2 inflammation, common in individuals with allergies, within the gastrointestinal tract. Immune cells related to allergies, including mast cells, are activated, and tissues exposed to the air can become inflamed. Increased IL33, a protein released by stromal cells, also activates group 2 innate lymphoid cells (ILC2s).
The study by Artis and colleagues, published in Nature this year, links the immune system to the microbiota in the gastrointestinal tract and lungs. Comparative models and human cell lines were used in this study to establish the translational power of these findings. Artis and others demonstrated altered microbiota, leading to bile acid changes. Bile acids were increased with an inulin-fiber-induced microbiota shift in mice. Consequently, the bile acids increased the release of IL33, which activated ILC2s. The activation of ILC2s leads to type 2 inflammation in both murine and human microbiota. The researchers showed that cholic acid, a type of bile acid found in humans, mimics type 2 inflammation induced by inulin. Moreover, the deletion of the bile acid receptor, farnesoid X receptor, negated the effects of inulin. Artis and colleagues further showed that genetically deleting bile-acid-metabolizing enzymes suppress inulin-induced type 2 immunity. However, the mechanism investigated is inulin-dependent; therefore, other fiber diets may affect the microbiota differently.
Artis and colleagues clearly state how an inulin diet can shift the microbiota and change the metabolites in the peripheral blood throughout the body. Overall, Artis and colleagues were the first to define the mechanism by which an inulin diet alters the microbiota and its metabolites. They highlight the complex environment within the gastrointestinal tract and how the immune system responds to diet. Finally, they indicate that understanding the mechanisms of microbiota-dependent bile acids can provide insight into therapeutic strategies.
Nature, Weill Cornell Medicine, David Artis, ILC2s, Study previously reported, Inulin
